Single-stage isolated DC-DC converters

ABSTRACT

According to one aspect of the present disclosure, a single-stage converter includes a rectifying circuit and a buck-boost circuit. The buck-boost circuit includes an inductor with a center tap configured to supply an output of the buck-boost circuit to the rectifying circuit. The buck-boost circuit also includes first and second interleaved arms arranged in parallel with a voltage input of the single-stage converter. The first and second interleaved arms are each coupled to the inductor and include a plurality of switches operable to control the output of the buck-boost circuit.

TECHNICAL FIELD

The present disclosure relates to power conversion and, moreparticularly, to single-stage isolated DC-DC converters.

BACKGROUND

Often times, electronics and other applications call for powercharacteristics that are different from available power sources.Converters, transformers, and/or combinations thereof address theproblem of mismatched power sources and power needs. Converters includeelectronic circuits and electromechanical devices that convert a sourceof direct current (DC) from one voltage level to another. A transformeris conventionally utilized to increase or decrease the alternatingvoltages in electric power applications. However, power conversion maybe costly, in terms of component size, expense, noise introduction,power consumption, thermal load, etc. Converters situated proximate anelectronics application, so-called point of load (POL) converters, areperhaps even more susceptible to these challenges. Further, increasinglywide input and/or output operational ranges are called for byapplications more and more frequently. Also, particularly for POLapplications, increased power density is desirable. As such,single-stage converters with wide gain range and efficient control, suchas those disclosed below, represent an improvement in the art.

The description provided in this background section should not beassumed to be prior art merely because it is mentioned in or associatedwith the background. The background section may include information thatdescribes one or more aspects of the subject technology.

SUMMARY

According to one aspect of the present disclosure, a single-stageconverter includes a rectifying circuit and a buck-boost circuit. Thebuck-boost circuit includes an inductor with a center tap configured tosupply an output of the buck-boost circuit to the rectifying circuit.The buck-boost circuit also includes first and second interleaved armsarranged in parallel with a voltage input of the single-stage converter.The first and second interleaved arms are each coupled to the inductorand include a plurality of switches operable to control the output ofthe buck-boost circuit.

According to another aspect of the present disclosure, a power circuitincludes a rectifier having a tank resonator, a transformer, and atleast one active component. In accordance with this aspect, the powercircuit also includes a buck-boost circuit having an inductor operableto generate an AC output to the rectifier, and interleaved arms coupledto the inductor such that the interleaved arms include a plurality ofswitches operable to control power supplied to the inductor.

Yet another aspect of the instant disclosure describes a method ofoperating a converter including operating a plurality of switchesdisposed in first and second interleaved arms of the converter togenerate an AC voltage in an inductor of the converter that is coupledto the first and second interleaved arms, and rectifying the AC voltagewith an LLC resonant circuit of the converter and at least one diode ofthe converter.

Other aspects and advantages of the present disclosure will becomeapparent upon consideration of the following detailed description andthe attached drawings wherein like numerals designate like structuresthroughout the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below refers to the appended drawings, inwhich:

FIG. 1 is a schematic circuit diagram illustrating a single-stageconverter capable of developing a wide gain range;

FIG. 2 is a chart representing operational modes and switching patternsof the single-stage converter of FIG. 1 and output voltages resultingtherefrom;

FIG. 3 is a chart representing a differential mode output for a dutycycle of less than 50% and circuit diagrams representing varying outputsof the differential mode;

FIG. 4 is a chart representing a differential mode output for a dutycycle of greater than 50% and circuit diagrams representing varyingoutputs of the differential mode;

FIG. 5 is a chart representing a common mode output for a duty cycle ofless than 50% and circuit diagrams representing varying outputs of thecommon mode;

FIG. 6 is a chart representing a common mode output for a duty cycle ofgreater than 50% and circuit diagrams representing varying outputs ofthe common mode;

FIG. 7 illustrates several schematic circuit diagrams of thesingle-stage converter with modified buck-boost circuit arrangements;and

FIG. 8 illustrates several schematic circuit diagrams of thesingle-stage converter with modified rectifier circuit arrangements.

In one or more implementations, not all of the depicted components ineach figure may be required, and one or more implementations may includeadditional components not shown in a figure. Variations in thearrangement and type of the components may be made without departingfrom the scope of the subject disclosure. Additional components,different components, or fewer components may be utilized within thescope of the subject matter disclosed.

DETAILED DESCRIPTION OF THE DRAWINGS

The detailed description set forth below is intended as a description ofvarious implementations and is not intended to represent the onlyimplementations in which the subject technology may be practiced. Asthose skilled in the art would realize, the described implementationsmay be modified in various different ways, all without departing fromthe scope of the present disclosure. Still further, modules andprocesses depicted may be combined, in whole or in part, and/or divided,into one or more different parts, as applicable to fit particularimplementations without departing from the scope of the presentdisclosure. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive.

Conventionally, two-stage solutions including a non-isolated buck-typeor boost-type converter and an LLC DC transformer have been widelyadopted for POL and similar applications in order to facilitateincreased step-up/step-down conversion and relatively wide voltage gainranges. However, conventional two-stage solutions require complexcircuitry. This increased circuit complexity results in practicallimitations on power density, efficiency, and expense. With reference toFIGS. 1-8, a single-stage converter 100 embodied as an integratedbuck-boost LLC converter comprising a buck-boost circuit 102 and arectifier circuit 104 (including, for example, resonant tank(s), LLCresonant circuit, LC circuit, CLLC circuit, rectifying diode(s),synchronous rectifier(s), and/or other suitable electrical components)is shown and described. The single-stage isolated DC-DC converter 100features increased efficiency, greater power density, and wide-rangevoltage gain. An extended operational range of the single-stageconverter 100 facilitates use of the single-stage converter 100 forDC-DC power conversion requiring high power density and wide-rangingvoltage conversion ratios. For example, the single-stage converter 100is desirable for electric vehicle charging, POL power supplies, andother similar applications. The buck-boost circuit 102 comprises amultiplexing switching cell (MSC) 106 that provides soft switching.Additionally, in an example embodiment, the single-stage converter 100integrates all or most of the magnetic components disposed therein.Integration of the magnetic components may result in reduced transformerand/or inductor size for the single-stage converter 100. However,unsymmetrical current stress may be applied across the MSC 106.

One illustrative embodiment of the single-stage converter 100 and basicoperational principles thereof are shown in FIGS. 1 and 2-6,respectively. Specifically, the example single-stage converter 100diagrammed in FIG. 1 is a half-bridge LLC converter configuration 108(see also FIG. 8). Referring now to FIG. 1, first and second arms 110,112 of the buck-boost circuit 102 are connected to a voltage inputV_(in). The first arm 110 comprises a first capacitor C1 and a firsthalf-bridge 114. The first half-bridge 114 further comprises first andsecond complementary switches S1, S2 disposed about a first node 116.The first capacitor C1 is arranged in series with the first switch S1 ofthe first half-bridge 114.

The second arm 112 comprises a second capacitor C2 and a secondhalf-bridge 118. The second half-bridge 118 comprises third and fourthcomplementary switches S3 and S4 disposed about a second node 120. Thesecond capacitor C2 is arranged in series with the third switch S3 ofthe second half-bridge 118. In this example embodiment, the first andsecond arms 110, 112 are interleaved (i.e., phase shifted by 180°). Thefirst and third switches S1, S3 may operate according to a duty cycle D(see FIGS. 2-6). Further, the first and second half-bridges 114, 118 arecoupled to a DC inductor L_(dc) via the first and second nodes 116, 120of said respective first and second half-bridges 114, 118. It iscontemplated that switches, including the first through fourth switchesS1-S4 may include one or more diodes and or other activeelectronic/semiconductor components. In example embodiments, ultra-fastdevices, such as silicon carbide (SiC) and/or gallium nitride (GaN)diodes, may be used as the switches S1-S4 disposed within the buck-boostcircuit 102.

The DC inductor L_(dc) comprises a center tap 122 generating a single ACoutput of the buck-boost circuit 102. In this example configuration, acommon mode output voltage 124 (see FIGS. 5 and 6) of the first andsecond arms 110, 112 develops an AC output voltage, and a differentialmode output voltage 126 (see FIGS. 3 and 4) of the two arms 110, 112develops pulse voltages across the DC inductor L_(dc). According to thisconfiguration, the non-inverting buck-boost-type converter 102 isintegrated between the first and second arms 110, 112. The buck-boostcircuit 102 develops a switched-mode power supply across the inductorL_(dc). Therefore, the inductor L_(dc) operates according to thevolt-second balancing of magnetic components. The relationship betweenvoltage and the duty cycle D across the inductor L_(dc) is:V _(in) +V _(c)=0.5*V _(in) /D

The interleaved first and second arms 110, 112 may then be used as aphase leg to construct half-bridge, single-phase, and/or multi-phase LLCconverters. As noted above, the example embodiment diagrammed in FIG. 1is the half-bridge LLC converter configuration 108. A current withineach of the first and second half-bridges 114, 118 is unsymmetrical. Asa result, each of the first and third switches S1, S3, respectively inseries with the first and second capacitors C1, C2, carries only ACcurrent. As a result, an AC output voltage of the buck-boost circuit 102is symmetrical.

Referring still to FIG. 1, the buck/boost circuit 102 is operativelycoupled with the rectifier circuit 104. The rectifier circuit 104 isconfigured between the center tap 122 of the DC inductor L_(dc) and thevoltage input V_(in). The rectifier circuit 104 may be an LLC resonantconverter/tank resonator comprising a resonant inductor L_(r) and aresonant capacitor C_(r) disposed on either side of a transformer T. Theresonant inductor L_(r) is operatively coupled to the center tap 122 andreceives, as the input thereto, the AC output of the buck/boost circuit102. The resonant capacitor C_(r) is operatively coupled to the voltageinput V_(in). One or more rectifying diodes 134 may be operativelycoupled between the transformer T and an output capacitor Co, and/or aresistive load R, and/or another suitable load. The present disclosurecontemplates a rectifying power supply topology comprising at least onediode and one capacitor. In the example shown in FIG. 1, a singlerectifying diode 134 a rectifies one half cycle of the AC signal. Also,in example embodiments, one or more synchronous rectifiers, which mayinclude active electrical components such as one or more metal oxidesemiconductor field-effect transistor(s) (MOSFETs), bipolar junctiontransistor(s) (BJTs), and/or other suitable transistor(s), may becombined with or substituted for the one or more rectifying diodes 134.As a result, the output capacitor C_(o) may be relatively large tosuitably filter and/or compensate the output DC signal and desirablyprevent ripple in the output voltage. In example embodiments, switchingdevices (e.g., mechanical switches, high-speed switches, other suitableactive component switches, etc.) may replace the one or more diodes 134in the rectifier circuit 104.

Referring now to FIGS. 2-6, operation of the single-stage converter 100facilitates increased voltage gain and straightforward control. Thesingle-stage converter 100 may be operated according to a pulse widthmodulation (PWM) mode, a pulse frequency modulation (PFM) mode, and/or ahybrid mode combining PWM and PFM control schemes. The voltage gain maybe regulated by modification of the duty cycle D. In the exampleembodiment of FIG. 1, the AC voltage applied to the rectifier circuit104 is:0.5*(Vin+Vc)=0.25*Vin/D

In an example with the duty cycle D selected as a 50% duty cycle, thesingle-stage converter 100 develops unit voltage gain similar to aconventional half-bridge LLC circuit. However, when an exampleembodiment operates with the duty cycle D set at other than 0.5 (i.e.,other than 50%), then the AC voltage across the rectifier circuit 104develops a gain of 0.5/D. For example, when the duty cycle D changesbetween 0.25 and 0.75, the integrated buck-boost LLC converter 100provides voltage gain between 2 and ⅔, at the respective duty cycles D.Additionally, further voltage gain may be obtained by regulatingswitching frequency. If the single-stage converter 100 developsadditional voltage gain by a factor of 1 to 2 through PFM, then thecumulative voltage gain may range from about ⅔ to about 4. Therefore,this example represents a voltage gain range of about 6 or greater. Sucha relatively wide gain is desirable for any number of modernapplications discussed in the examples throughout this disclosure.Further, duty cycle changes and switching frequency control may beoptimized to ensure smooth transition between switching states.

Moreover, the single-stage converter 100 may operate according tozero-voltage turning-on (ZVS) for all the primary switching devices(e.g., first through fourth switches S1-S4) and zero-current turning-off(ZCS) for all the secondary devices (e.g., the one or more diodes 134)within the full operational range, as shown in FIG. 2. This results inlower voltage device ratings compared to conventional two-stagesolutions. Additionally, the single-stage converter 100 developssymmetrical AC voltage as compared with conventional buck-boost and LLCresonator cascaded converters.

Referring once again to FIG. 2, ZVS switching points for the first andsecond switches S1, S2 are indicated. Further, ZCS switching points forthe one or more rectifying diodes 134 are also illustrated in FIG. 2.Here, the ZVS and ZCS switching points are indicated when the duty cycleD is below 50% and when the duty cycle D is above 50%. Additionally,single-stage integration of the buck-boost circuit 102 with therectifier circuit 104 according to differential-mode and common-modevoltage gain outputs is facilitated by the interleaved arms 110, 112 andswitching arrangement thereof.

FIGS. 3 and 4 illustrate integration of the buck-boost circuit 102 bythe interleaved arms 110, 112 according to a differential-mode gain ofthe converter 100 (i.e., differences between the voltages in the firstand second arms 110, 112 are amplified). In FIG. 3, voltages through thefirst and second arms 110, 112 of the single-stage converter 100 areshown when the duty cycle D is less than 50%. When the duty cycle D islower than 50%, the inductor L_(dc) is charged through the second switchS1 of the first arm 110 and the fourth switch S4 of the second arm 112.FIG. 4 depicts voltages through the first and second arms 110, 112 whenthe duty cycle D is greater than 50%. Accordingly, the inductor L_(dc)is charged through the first capacitor C1 and the first switch S1 of thefirst arm 110 in connection with the second capacitor C2 and the thirdswitch S3 of the second arm 112 when the duty cycle is greater than 50%.

The inductor L_(dc) is discharged according to the same topologieswhether the duty cycle D is less or greater than 50%. Particularly, theinductor L_(dc) is discharged through the first capacitor C1 and thefirst switch S1 from the first arm 110 in connection with the fourthswitch S4 from the second arm 112. Also, the inductor L_(dc) isdischarged by the second switch S2 of the first arm 110 in connectionwith the second capacitor C2 and the third switch S3 of the second arm112.

The voltage diagrams of FIGS. 3 and 4 indicate charging and dischargingstates during the progression of the duty cycle D. In FIG. 3, stage Iindicates charging while stages III and IV indicate discharging.However, FIG. 4 illustrates charging at stage II while stages III and IVindicate discharging.

FIGS. 5 and 6 further illustrate integration of the buck-boost circuit102 by the interleaved arms 110, 112 according to a common-mode gain ofthe converter 100 (i.e., amplification of voltages present on both ofthe arms 110, 112 relative to the common/ground). In FIG. 5, the neutralstate, with the duty cycle D being less than 50%, is shown as activatingthe second switch S2 and the fourth switch S4. While in FIG. 6, theneutral state when the duty cycle D is greater than 50% activates thefirst capacitor C1 and the first switch S1 of the first arm 110 incorrespondence with the second capacitor C2 and the third switch S3 ofthe second arm 112.

FIGS. 5 and 6 diagram the same topologies for the inverting andnoninverting states, stages III and IV respectively, whether the dutycycle D is less or greater than 50%. The voltage diagrams of FIGS. 5 and6 indicate changing between neutral, inverting, and noninverting stagesduring the progression of the duty cycle D. In FIG. 5, stage I indicatesa neutral state while stages III and IV indicate inverting andnoninverting states, respectively. Further, in the diagram of FIG. 6,the neutral state is II and the inverting and noninverting statescorrespond to stages III and IV.

Referring now to FIGS. 7 and 8, several embodiments and variations ofthe single-stage converter 100 are illustrated. In FIG. 7, half-bridgetype DC-DC converters 200 a, 200 b are diagrammed each with the singlebuck-boost circuit 102. Also, in FIG. 7, single-phase DC-DC converters200 c, 200 d are shown with the half-bridge buck-boost circuit 102 incombination with a hybrid buck-boost phase circuit 202 comprising fifthand sixth switches S5, S6. The diagrams of (e) and (f) in FIG. 7 aresingle-phase or multiphase DC-DC converters 200 e, 200 f with two (or,in example embodiments, a plurality) of the buck-boost phase circuits102. In the example circuit configurations 200 e, 200 f, the capacitorsin each of the buck-boost phase circuits 102 may be disposed eitherseparately or in parallel. Each of the circuit configurations 200 a-200f of FIG. 7 further comprise an isolated rectifier circuit 204. Theisolated rectifier circuit 204 may be one of the LLC resonantconverters/circuit 104 or another suitable non-resonant converter (e.g.,full-bridge phase shift converter, dual-active-bridge converter, etc.).

In FIG. 8, example circuit configurations comprising the buck-boostcircuit 102 are combined with half-bridge LLC converters. The circuitconfiguration of 300 a is the single-stage converter 100 in thehalf-bridge LLC converter configuration 108 (see also FIG. 1). Further,a circuit configuration 300 b includes first and second resonantcapacitors C_(r1), C_(r2) operatively coupled in parallel with one endof the transformer T. This adds an additional capacitor as compared withthe resonant capacitor C_(r) of the LLC resonant circuit 104 illustratedin FIG. 1. In a circuit configuration 300 c, the first and secondresonant capacitors C_(r1), C_(r2) are operatively coupled to either endof the transformer T while the resonant inductor L_(r) is operativelycoupled to a center tap 302 of a primary winding 304 of the transformerT.

The embodiment(s) detailed above may be combined, in full or in part,with any alternative embodiment(s) described.

INDUSTRIAL APPLICABILITY

The above disclosure may represent an improvement in the art byproviding a reliable, efficient, and economical single-stage, wide-gainconverter. Compared to prior art two-stage converters with similarvoltage range capabilities, the subject matter of the instant disclosureoffers greater power density and straightforward control. Thesingle-stage converter may allow for a wide-gain range with fewer andsmaller electrical components.

While some implementations have been illustrated and described, numerousmodifications come to mind without significantly departing from thespirit of the disclosure, and the scope of protection is only limited bythe scope of the accompanying claims.

To the extent that the term include, have, or the like is used, suchterm is intended to be inclusive in a manner similar to the termcomprise as comprise is interpreted when employed as a transitional wordin a claim. Relational terms such as first and second and the like maybe used to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions.

Phrases such as an aspect, the aspect, another aspect, some aspects, oneor more aspects, an implementation, the implementation, anotherimplementation, some implementations, one or more implementations, anembodiment, the embodiment, another embodiment, some embodiments, one ormore embodiments, a configuration, the configuration, anotherconfiguration, some configurations, one or more configurations, thesubject technology, the disclosure, the present disclosure, othervariations thereof and alike are for convenience and do not imply that adisclosure relating to such phrase(s) is essential to the subjecttechnology or that such disclosure applies to all configurations of thesubject technology. A disclosure relating to such phrase(s) may apply toall configurations, or one or more configurations. A disclosure relatingto such phrase(s) may provide one or more examples. A phrase such as anaspect or some aspects may refer to one or more aspects and vice versa,and this applies similarly to other foregoing phrases.

The disclosed systems and methods are well adapted to attain the endsand advantages mentioned as well as those that are inherent therein. Theparticular implementations disclosed above are illustrative only, as theteachings of the present disclosure may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. Furthermore, no limitationsare intended to the details of construction or design herein shown,other than as described in the claims below. It is therefore evidentthat the particular illustrative implementations disclosed above may bealtered, combined, or modified and all such variations are consideredwithin the scope of the present disclosure. The systems and methodsillustratively disclosed herein may suitably be practiced in the absenceof any element that is not specifically disclosed herein and/or anyoptional element disclosed herein.

It should be understood that the described instructions, operations, andsystems can generally be integrated together in a singlesoftware/hardware product or packaged into multiple software/hardwareproducts.

The use of the terms “a” and “an” and “the” and “said” and similarreferences in the context of describing the subject matter of thepresent disclosure (especially in the context of the following claims)are to be construed to cover both the singular and the plural, unlessotherwise indicated herein or clearly contradicted by context. Anelement proceeded by “a,” “an,” “the,” or “said” does not, withoutfurther constraints, preclude the existence of additional same elements.Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the disclosure and does not pose alimitation on the scope of the disclosure unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the disclosure.

The invention claimed is:
 1. A method of operating a converter, themethod comprising: operating a plurality of switches disposed in firstand second interleaved arms of the converter to generate an AC voltagein an inductor of the converter that is coupled to the first and secondinterleaved arms; and supplying the AC voltage to an LLC resonantcircuit of the converter from a center tap output of the inductor, theinductor in parallel with a resonant capacitor of the LLC resonantcircuit.
 2. The method of claim 1, wherein operating the plurality ofswitches comprises zero-voltage switching of the plurality of switches.3. The method of claim 1, wherein operating the plurality of switchescomprises controlling switching of the plurality of switches using pulsewidth modulation.
 4. The method of claim 1, wherein operating theplurality of switches comprises controlling switching of the pluralityof switches using frequency modulation.
 5. The method of claim 1,further comprising: rectifying the AC voltage with the LLC resonantcircuit of the converter and at least one diode of the converter.
 6. Apower circuit comprising: a rectifier; and a buck-boost circuitcomprising: an inductor operable to generate an AC output to therectifier; and interleaved arms coupled to the inductor, the interleavedarms comprising a plurality of switches operable to control powersupplied to the inductor; wherein each of the interleaved armscomprises: at least two switches of the plurality of switches directlyconnected in series; at least one capacitor in series with the at leasttwo switches; and a node disposed between the at least two switches andcoupled to the inductor of the buck-boost circuit.
 7. The power circuitof claim 6, wherein the plurality of switches are operable by one ofpulse width modulation and frequency modulation to control a voltagegain of the power circuit.
 8. The power circuit of claim 6, wherein theinterleaved arms comprise (i) a first interleaved arm coupled to a firstend of the inductor and (ii) a second interleaved arm coupled to asecond end of the inductor.
 9. The power circuit of claim 8, wherein afirst node of the first interleaved arm is coupled to a first side ofthe inductor and a second node of the second interleaved arm is coupledto a second side of the inductor.
 10. The power circuit of claim 6,wherein the inductor of the buck-boost circuit is operable to generatethe AC output through a center tap of the inductor, the center tapcoupled to another inductor of the rectifier.
 11. The power circuit ofclaim 6, wherein the rectifier comprises a tank resonator.
 12. The powercircuit of claim 11, wherein the rectifier further comprises at leastone of a rectifying diode and a synchronous rectifier.
 13. The powercircuit of claim 11, wherein the rectifier further comprises at leastone rectifying diode operatively coupled to a filtering outputcapacitor.
 14. A system, comprising: a buck-boost circuit, comprising:an inductor operable to generate an AC output; and interleaved armscoupled to the inductor, the interleaved arms comprising a plurality ofswitches operable to control power supplied to the inductor; whereineach of the interleaved arms comprises: at least two switches of theplurality of switches directly connected in series; at least onecapacitor in series with the at least two switches; and a node disposedbetween the at least two switches and coupled to the inductor.
 15. Thesystem of claim 14, wherein the plurality of switches are operable byone of pulse width modulation and frequency modulation to control avoltage gain of the buck-boost circuit.
 16. The system of claim 14,wherein the interleaved arms comprise (i) a first interleaved armcoupled to a first end of the inductor and (ii) a second interleaved armcoupled to a second end of the inductor.
 17. The system of claim 16,wherein a first node of the first interleaved arm is coupled to a firstside of the inductor and a second node of the second interleaved arm iscoupled to a second side of the inductor.
 18. The system of claim 14,wherein the inductor of the buck-boost circuit is operable to generatethe AC output through a center tap of the inductor.
 19. The system ofclaim 14, further comprising: an LLC resonant circuit configured toreceive the AC output from the inductor, wherein the inductor is inparallel with a resonant capacitor of the LLC resonant circuit.
 20. Thesystem of claim 19, wherein the inductor includes a center tap outputconfigured to supply the AC output to the LLC resonant circuit.